All electronics with a fan (or even without one) collect dust over time.

This dust can eventually clog the heatsinks and/or fans, causing your Xbox 360 to be noisy or overheat.

Follow this guide up to (and including) step 35. Clean the fans, motherboard, and all of the casing with high purity (90+%) isopropyl alcohol, and blow out the dust from the heat sinks with compressed air. Then reassemble your Xbox 360.